1. Field of the Invention
The present invention relates to a magneto-optical recording medium capable of performing an over-write operation by modulating a light beam.
2. Related Background Art
As a conventional technique for performing an over-write operation on a magneto-optical recording medium, an optical modulation method (U.S. Pat. No. 5,132,945 or the like) using an exchange-coupled multilayered film or a magnetic modulation method is known. The optical modulation method is more advantageous than the magnetic field modulation method since it allows high-speed modulation, and the use of double-sided media.
An optical modulation over-write method using an exchange-coupled multilayered film is basically attained by a two-layered film comprising first and second magnetic layers. In principle, this method consists of a first type recording process for setting the second magnetic layer in an erased state in advance, and orienting the direction of magnetization of the first magnetic layer in a stable coupling state with respect to the second magnetic layer by a heating/cooling process to a first type temperature state, and a second type recording process for performing recording on the second magnetic layer using, e.g., a bias magnetic field by a heating/cooling process to a second type temperature state, and at the same time, transferring this state to the first magnetic layer. These two types of temperature states are generated in correspondence with modulation of laser power, thereby attaining an over-write operation.
In order to set the second magnetic layer in an erased state in advance, in general, an initialization magnetic field is applied to the second magnetic layer before recording so as to magnetize only the second magnetic layer in the direction of the magnetic field. For this purpose, the coercive force, at room temperature, of the second magnetic layer is set to be smaller than that of the first magnetic layer. In this case, a magnetization device for generating a magnetic field of at least about 2 kOe is required, and a recording apparatus becomes undesirably complicated.
For this reason, as another means for setting the second magnetic layer in an erased state in advance, the following method has been proposed in, e.g., Japanese Laid-Open Patent Application No. 63-268103. In this method, a third magnetic layer which maintains an erased initial state all the time even after the above-mentioned recording operation is stacked on and exchange-coupled to the second magnetic layer. Upon completion of the recording process, the direction of magnetization of the second magnetic layer is oriented in a stable direction with respect to the coupling action to the third magnetic layer, thereby setting the second magnetic layer in an erased state.
Furthermore, still another method has been proposed. That is, the second magnetic layer is removed from the above-mentioned structure, the direction of magnetization of the first magnetic layer is oriented in a stable coupling state with respect to the third magnetic layer by the heating/cooling process to the first type temperature state by using a temperature gradient in the direction of film thickness or the like so as to set the first magnetic layer in an erased state, recording on the first magnetic layer is performed by the heating/cooling process to the second type temperature state, and the recorded state is held.
As a general problem posed upon execution of an optical modulation over-write operation, initially recorded information tends to remain unerased. As one cause of this problem, the width of a recording/erasing region in the over-write operation is decreased to be smaller than that of a region where initial recording information is recorded due to, e.g., a variation of recording laser power, or a recording/erasing region in the over-write operation is shifted from a region where initial recording information is recorded due to, e.g., a tracking control error.